1. Field of the Invention
The present invention relates to an image recording apparatus and an image recording method, and more particularly to image correction processing technology which is suitable for correcting density variations caused by variation in characteristics among a plurality of recording elements in a recording head.
2. Description of the Related Art
An image recording apparatus (inkjet printer) has been used which includes an inkjet type of recording head having a plurality of ink ejection ports (nozzles). In this type of image recording apparatus, problems of image quality are liable to arise due to the occurrence of density variations (density non-uniformities) in the recorded image caused by variations in the ejection characteristics of the nozzles. FIG. 21 is an illustrative diagram showing a schematic view of examples of variations in the ejection characteristics of the nozzles, and density variations appearing in recording results.
In FIG. 21, reference numeral 300 represents a line head, reference numeral 302-i (where i=1 to 8) represents a nozzle, reference numeral 304-i (i=1 to 8) represents a dot formed by a droplet ejected from the nozzle 302-i (i=1 to 8). Here, it is supposed that the recording medium, such as recording paper, is conveyed in a direction perpendicular to the breadthways direction of the line head 300 (the nozzle arrangement direction) (namely, in the direction of arrow S), and the nozzle arrangement direction in the line head 300 is taken to be the main scanning direction, while the direction of relative conveyance of the recording medium with respect to the line head 300 (the direction S) is taken to be the sub-scanning direction.
In the example shown in FIG. 21, a depositing position error occurs at the nozzle 302-3, which is third from the left (namely, the droplet ejected from the nozzle 302-3 deposits on the recording medium at a position diverging from the originally intended depositing position in the leftward direction in FIG. 21), and a droplet volume error occurs at the sixth nozzle 302-6 (namely, the droplet ejected from the nozzle 302-6 has a greater droplet volume than the originally intended volume). In this case, density non-uniformity streaks occur at the positions in the print image corresponding to the nozzles 302-3 and 302-6 producing the depositing position error and the droplet volume error (namely, the positions indicated by A and B in FIG. 21).
In the case of a serial (shuttle) scanning type of image recording apparatus, which performs image recording by driving a recording head to scan a plurality of times over the prescribed print region, it is possible to avoid density non-uniformities by means of a commonly known multi-pass printing method, but in the case of a single pass system (line head system) which records images by means of a single scanning action, it is difficult to avoid density non-uniformities.
Since it is difficult to completely prevent variations in ejection characteristics among the nozzles in terms of the process of manufacturing the recording head, then various technologies for correcting the variations have been proposed (see, Japanese Patent Application Publication No. 2007-125877 and U.S. Pat. No. 7,484,824).
With the object of eliminating stripe-shaped non-uniformities (banding) caused by a so-called “flight deflection effect”, Japanese Patent Application Publication No. 2007-125877 discloses technology which changes the density of the pixels of the image data allocated to particular dot rows (namely, changing the dot size, dot pattern, or the like) if there is deviation in the interval between dot rows, so as to suppress non-uniformity streaks. According to this, the density range (L2) of the image data is determined so as not to use a region (region C) where banding cannot be corrected, as confirmed on an output density map (FIG. 13 of Japanese Patent Application Publication No. 2007-125877) relating to the amount of deviation in dot pitch.
U.S. Pat. No. 7,484,824 describes, in columns 10 to 14, outputting a test pattern, obtaining depositing position error data from the print results, using this depositing position error data to define a density profile D(x) which incorporates the error characteristics of respective nozzles, converting this density profile into a function T(f) by Fourier transform and then calculating a density correction coefficient by minimizing the low-frequency component of the power spectrum of this function.
However, with the technology described in Japanese Patent Application Publication No. 2007-125877, the density range of the image data is determined on the basis of information about the droplet depositing positions where there is greatest positional displacement, and hence this leads to decline in the resolution of the output image density. On the other hand, U.S. Pat. No. 7,484,824 does not mention density output limits, and therefore when calculating correction, abnormally large density values or abnormally small (or negative) density values may be derived. If the apparatus output is not able to handle the abnormal values thus calculated, then correction can not be complete.
In other words, in the case of a method which corrects for banding by using information about the droplet depositing positions, as in U.S. Pat. No. 7,484,824, then the interval between adjacent droplet depositing positions may become extremely large or extremely small (close to zero), depending on the depositing position error of the ejected droplets. On the other hand, since the output apparatus (printer) has a set output density range and is not able to output densities outside that range, then problems will occur if it is sought to output the corrected densities without taking this density range into account.
One possible method for resolving this method is to set an upper limit value and a lower limit value for the corrected density, and to restrict any values outside these limits to the limit values. However, in the case of this method, problems occur, such as the fact that the density of the image data (input density) and the picture (image contents) are not reflected accurately in the output, the density resolution declines, and so on.